Various pyrazolone derivatives are known as magenta dye forming couplers (hereinafter, referred to as a magenta coupler). However, pyrazolone derivatives generally used for photographic sensitive materials are 4-equivalent couplers. Such compounds theoretically require the development of 4 moles of silver halide as an oxidizing agent for forming 1 mole of a dye by reacting with an aromatic primary amine developing agent. On the contrary, pyrazolones having an active methylene group substituted by a group which can be released by oxidative coupling with an oxidation product of the primary amine developing agent require development of only two moles of silver halide. In addition, the 4-equivalent pyrazolone derivatives have a low color forming efficiency (conversion of the coupler into the dye) and form generally only a 1/2 mole or so of the dye per mole of the coupler.
As a means for improving the color forming efficiency by decreasing the amount of silver halide required for development, utilization of 2-equivalent pyrazolone magenta couplers has been proposed. Examples of pyrazolone derivatives which release an oxygen atom include compounds having an aryloxy group in the 4-position of 5-pyrazolone as described in U.S. Pat. No. 3,419,391 and compounds having an acyloxy group as described in U.S. Pat. Nos. 3,311,476 and 3,926,631. In these cases, the finally resulting 2-equivalent magenta couplers become remarkably expensive because they are synthesized by complicated steps. The use of such couplers is not desirable because the cost of producing sensitive materials with them does not decrease much even if the amount of silver halide or the amount of couplers is reduced by half.
Examples of pyrazolone derivatives which release a nitrogen atom include compounds having an imidazolyl group, a pyrazolyl group or a triazolyl group in the 4-position of 5-pyrazolone as described in U.S. Pat. Nos. 4,076,533 and 4,241,168. Synthesis of these couplers from 4-equivalent couplers which have no substituent on the coupling position requires 2 to 4 steps. Consequently, such couplers are expensive. In addition, photographic properties are sometimes damaged, because some of these couplers are unstable or have a low reaction rate for coupling with the oxidation product of aromatic primary amine.
Examples of pyrazolone derivatives which release a sulfur atom include compounds having a heterocyclic thio group or an arylthio group in the 4-position of 5-pyrazolone as described in U.S. Pat. No. 3,227,554, compounds having a thiocyano group as described in U.S. Pat. No. 3,214,437 and compounds having a dithiocarbamate group as described in U.S. Pat. No. 4,032,346. These compounds are advantageous in that most of them can be synthesized from 4-equivalent pyrazolones by one step. However, when they are incorporated in the silver halide sensitive materials, the mercapto compound released during the development reacts with the silver halide and resulted in inhibition of development. Furthermore, such a compound is generally very unstable. Consequently, it is difficult to use them for common uses and they are used only in special cases.